qj 6 Dr. Herschei/s Description of a 
settle upon them, and sometimes fill up a channel of the pul- 
leys, so that the least deviation from the perpendicular may 
throw a rope out of its place. Should this happen in the night, 
when it might not be immediately perceived, the rope would 
soon be injured, or even cut through, by the continuation of 
the force that acts upon it. Besid'es, this irregular motion of 
the pulleys, when the telescope is finely suspended in the me- 
ridian; will tend to produce a little deviation in right ascen- 
sion, which ought to be avoided. My pulleys, therefore, are 
all but one in a meridional situation, and this might also be 
turned the same way if there were occasion for it. The double 
pulleys are placed under each other ; by which means the 
stress of the lower ones at the top, and the upper ones at the 
bottom, adds to their meridional and perpendicular steadiness. 
In order to command every altitude, from the horizon to 
the zenith, it was necessary that the point of support should 
be moveable. Its motion is effected by a mechanism which I 
shall now explain. 
Eight bars, <i\ inches broad, and i|- thick, were cut into 
teeth at the distance of \\ inch each ; and afterwards con- 
nected by slips screwed against both sides of the places where 
two butt together. Their length is such, that four and four 
being joined make up two bars of 29 feet 8 inches long each. 
Two loops which are screw-bolted to the ends of them, take 
hold of the axle, fig. 34. at D and E, which in those places 
is made round for that purpose. 
Upon the foundation beams in fig. 3/are fixed four short cross 
beams, at 1 1 mm nn 00 ; these carry the following machine., 
A handle which turns a pinion of eight leaves, drives a wheel 
of 20 inches diameter, with 51 jteeth ; the axle of the wheel 
